CAPE CANAVERAL,
Fla. - It's been three long years since the loss of Columbia, but NASA's space shuttle fleet is
back in action after six astronauts rode their 100-ton Discovery orbiter home
safely Monday.

Discovery
and its STS-121 astronaut crew touched down here at NASA's Kennedy Space Center
(KSC) at 9:14 a.m. EDT (1314 GMT), ending a successful 13-day
mission and the last of two test flights following the 2003 Columbia tragedy.The
orbiter touched down after 202 trips around Earth.

Lindsey
said the superheated plasma encountered by the shuttle during reentry gave his
crew quite a show.

"We
could see the bright orange glow above and I could see the Earth moving below,"
Lindsey said. "It was spectacular. We could actually see the Moon through
the plasma."

Discovery rocketed
into space on July 4, a first-ever Independence Day
shuttle launch for NASA, almost one year after the agency's first post-Columbia
mission returned its aging orbiter fleet to flight. Lindsey and his crew spent eight
days resupplying the International Space Station (ISS), making crucial repairs
to the orbital laboratory and testing new heat shield inspection and repair
techniques.

NASA has
estimated its Columbia
investigation and return to flight efforts have cost about $2.3 billion through
2006.

"I think
we're back to space station assembly, to shuttle flights, but we're still going
to watch and we're still going to pay attention," Lindsey said earlier. "We're
never ever going to let our guard down."

Spotless
mission

By all
accounts from NASA mission managers, engineers and flight controllers,
Discovery's STS-121 mission has been the cleanest ever seen in the agency's
25-year history of shuttle flight.

"We've
inspected [the heat shield] more on STS-121 than on any other mission," said
Steve Stitch, NASA's reentry flight director, adding that analysts found no
sign of troubling damage to the orbiter's heat-resistant tiles and carbon
composite panels.

That clean
bill of health comes after an intense engineering job to pull as much unneeded
foam insulation as possible from the wrapping of Discovery's external tank,
including a large
ramp similar to one that shed a one-poundchunk of foam during the
shuttle's STS-114 launch in
July 2005.

A slightly
larger piece of foam about the size of a briefcase doomed Columbia and its crew when it fell from the
orbiter's external tank and gouged a hole in the shuttle's left wing leading
edge during a Jan. 16, 2003
launch. The damaged heat shield allowed searing hot atmospheric gases into Columbia's wing, leading
to its destruction.

But after a
major overhaul of shuttle external tanks - NASA shuttle chief Wayne Hale has
called the new rampless design the largest aerodynamic change since the first
shuttle launch in 1981 - the space agency can claim an undeniable success.

The largest area of foam shed during launch was about the
same as that of a legal-size
sheet of paper, weighed less than one ounce and fell off in six separate
pieces, NASA has said.

New cameras
mounted to the orbiter's solid rocket boosters gave unprecedented
- and stunning - viewsof Discovery's wing edges and tile-lined
belly during launch to track any foam debris loss, and an effort to keep pesky
vultures away from the shuttle's launch site after one struck the orbiter's
STS-114 external tank last year appears to have been successful.

"I hope our
legacy was that we closed out our goals of the post-Columbia flights," Lindsey
told reporters Sunday.

"There was
a lot to get done and we worked hard to make it all happen," Fossum said.

Among the
critical deliveries was German astronaut Thomas Reiter, who returned the ISS to
its full three-person capacity for the first time since the Columbia accident.

Meanwhile,
the entire crew worked together for meticulously choreographed inspections of
Discovery's entire heat shield using a sensor-laden boom attached to the end of
the orbiter's robotic arm.

"To me, the
most amazing thing was watching the footage from launch, looking at the early
inspection," said NASA astronaut Mark Polansky, commander of Discovery's next
flight - STS-116 to launch Dec. 14 - in an interview.

An ISS
gate opens

Discovery's
successful mission is the starting pistol for a marathon of ISS-bound shuttle
flights to first build up the station's power and support systems, and then
install new modules and laboratories that have been waiting for NASA's shuttle
fleet to resume construction.

Up next is
NASA's STS-115 shuttle mission, slated to launch between Aug. 28 and Sept. 5,
to deliver a 17.5-ton solar array to the ISS and truss segment. A third shuttle
orbiter flight slated for 2006, Polansky's STS-116 command, will install the new
solar array along with another truss segment.

"Every
mission is the critical one," Polansky said. "Every mission depends upon the
successful completion of the previous one...I find it exciting."